Vplan Cleanup

Created by: silabs-robin

This PR does 4 vplan cleanups.

1. Delete "instruction_groups" vplan.
   • (More info below.)
2. Generate CSV and JSON files.
   • (A future PR must deal with S vs X discrepancies.)
3. Delete unused "custom_circuitry" directory.
   • (It has never been used.)
4. Delete old "external debugger" vplan.
   • (More info below.)

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"instruction_groups" vplan is redundant. See its content in the screenshot below. (It also doesn't say what to do.)

It is redundant because the same content lives in the cvverif top-level vplan RV32ISA_Sequential. See the second screenshot to see that it covers the same "instruction groups".

instruction_groups:

RV32ISA_Sequential:

The link to coverage in RV32ISA_Sequential ultimately points to this definition which maches the instruction_groups vplan descriptions:

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External debugger vplan is redundant too.

I checked all its content vs the normal debug vplan and found only 2 things unique to the external debugger vplan. Firstly, legal use of dscratch* (see https://github.com/openhwgroup/core-v-verif/pull/2276#discussion_r1381910015). Secondly, "semihosting" which is irrelevant.